Method for multicast frame transmission and duplicated multicast frame detection

ABSTRACT

A method and apparatus of transmitting a multicast frame in a wireless communication system is provided. The method comprises transmitting a request to send (RTS) frame to stations (STAs) included in a multicast group by using an omni-directional antenna, and receiving a clear to send (CTS) frame transmitted by the STAs included in the multicast group in response to the RTS frame, and transmitting the multicast frame to the STAs included in the multicast group by using a directional antenna.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2009/00005942, filed on Oct. 15,2009, which claims the benefit of earlier filing date and right ofpriority to Korean Application Nos. 10-2009-0097492, filed on Oct. 14,2009 and 10-2008-0132451, filed on Dec. 23, 2008, and also claims thebenefit of U.S. Provisional Application Ser. Nos. 61/158,737, filed onMar. 9, 2009 and 61/105,429, filed on Oct. 15, 2008, the contents ofwhich are incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless local area network (WLAN),and more particularly, to a multicast/broadcast procedure in a very highthroughput (VHT) WLAN system.

BACKGROUND ART

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted. In the initial WLAN technology, a frequency of 2.4 GHz wasused according to the IEEE 802.11 to support a data rate of 1 to 2 Mbpsby using frequency hopping, spread spectrum, infrared communication,etc. Recently, the WLAN technology can support a data rate of up to 54Mbps by using orthogonal frequency division multiplex (OFDM). Inaddition, the IEEE 802.11 is developing or commercializing standards ofvarious technologies such as quality of service (QoS) improvement,access point protocol compatibility, security enhancement, radioresource measurement, wireless access in vehicular environments, fastroaming, mesh networks, inter-working with external networks, wirelessnetwork management, etc.

In the IEEE 802.11, the IEEE 802.11b supports a data transfer rate of upto 11 Mbps by using a frequency band of 2.4 GHz. The IEEE 802.11acommercialized after the IEEE 802.11b uses a frequency band of 5 GHzinstead of the frequency band of 2.4 GHz and thus significantly reducesinfluence of interference in comparison with the very congestedfrequency band of 2.4 GHz. In addition, the IEEE 802.11a has improvedthe data transfer rate to up to 54 Mbps by using the OFDM technology.Disadvantageously, however, the IEEE 802.11a has a shorter communicationdistance than the IEEE 802.11b. Similarly to the IEEE 802.11b, the IEEE802.11g implements the data transfer rate of up to 54 Mbps by using thefrequency band of 2.4 GHz. Due to its backward compatibility, the IEEE802.11g is drawing attention, and is advantageous over the IEEE 802.11ain terms of the communication distance.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data transfer rate which has been considered as adrawback in the WLAN. The IEEE 802.11n is devised to increase networkspeed and reliability and to extend an operational distance of awireless network. More specifically, the IEEE 802.11n supports a highthroughput (HT), i.e., a data processing rate of up to 540 Mbps, and isbased on a multiple input and multiple output (MIMO) technique whichuses multiple antennas in both a transmitter and a receiver to minimizea transmission error and to optimize a data rate. In addition, thisstandard may use a coding scheme which transmits several duplicatecopies to increase data reliability and also may use the OFDM to supporta higher data rate.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. A very high throughput (VHT) WLAN systemis one of IEEE 802.11 WLAN systems which have recently been proposed tosupport a data processing rate of 1 Gbps or higher. The VHT WLAN systemis named arbitrarily. To provide a throughput of 1 Gbps or higher, afeasibility test is currently being conducted for the VHT system whichuses 4?4 MIMO and a channel bandwidth of 80 MHz or higher.

As a mechanism for achieving a throughput of 1 Gbps or higher for theVHT WLAN, two methods are currently discussed, that is, a method ofusing a band of 6 GHz or lower and a method of using a band of 60 GHz.Among them, the method of using a channel of the band of 60 GHz isdrawing more attention. This is caused by the fact that a channel usingthe band of 6 GHz or lower is in use also by other wirelesscommunication systems, and thus available radio resources are limited.Such a disadvantage can be overcome by using a channel with the band of60 GHz. However, the band of 60 GHz has a demerit in that its servicecoverage is narrower than that of the band of 6 GHz or lower accordingto a feature of high frequency. Therefore, there is a need for a methodfor solving the narrow service coverage in a VHT WLAN system using theband of 60 GHz.

Meanwhile, data transmission of the WLAN system can be classified intounicast, multicast, and broadcast according to the number of targetdevices or destination devices. Unlike in the unicast where adestination device of transmit (Tx) data is a single terminal, thedestination device of the Tx data is a plurality of terminals in themulticast and the broadcast. In the multicast, a target address or adestination address of a Tx frame is specified as a multicast groupaddress. The broadcast is special multicast in which the multicast groupaddress specifies all terminals. Therefore, when simply referred to as‘multicast’ in the following description, it will be interpreted suchthat ‘broadcast’ is also included unless it is not allowed by nature.

Multicast transmission delivers a single data stream simultaneously to aplurality of destination terminals, and thus data traffic can be reducedand a channel can be effectively used. Such multicast can be useful toprovide a variety of information such as various applications, e.g.,video conference, corporate communication, distance learning, softwaredistribution, stock quotes, news, etc. Further, the multicast can alsobe used for a game played by multiple users over a wireless home networkor for an application which shares streaming data.

The multicast is based on the concept of a multicast group, that is, agroup of recipient terminals having an interest in a specific datastream. The terminals having an interest in receiving data to bemulticast have to be first registered to the multicast group in order toreceive the data. In a medium access control (MAC) layer, the multicastgroup is specified by a multicast MAC address. In general, a higherlayer than the MAC layer takes a charge of generation, registration,deregistration, and change of the multicast group. Those issues ofgeneration, registration, or the like of the multicast group specifiedby the MAC address are irrelevant to the present invention, and thusdescription thereof will be omitted.

In multicast transmission, it is difficult to confirm whether a terminalregistered to a specific multicast group, i.e., a destination terminal,has successfully received all pieces of data provided from a sourceterminal. In particular, the institute of electrical and electronicengineers (IEEE) 802.11 standard neither specifies an error recoverymechanism for multicast traffic nor provides any definition on a methodcapable of avoiding collision between a multicast frame and anotherframe. Therefore, a current multicast service does not guarantee thatthe destination terminal can completely and reliably receive data to bemulticast.

A method of using an adaptive modulation scheme is taken intoconsideration as one method of guaranteeing reliability of multicasttransmission. According to the adaptive modulation scheme, multicasttransmission is achieved by a possible lowest modulation scheme so thatall terminals registered to a corresponding multicast group can receivea multicast frame. However, the use of a low modulation scheme resultsin deterioration of a data transfer rate, which may impair improvementof a data throughput of a WLAN system.

DISCLOSURE OF INVENTION Technical Problem

The present invention provides a multicast method capable of not onlyexpanding service coverage but also ensuring reliability of multicasttransmission in a wireless local area network (WLAN) system.

The present invention also provides a method of detecting a framereceived in a duplicate manner by a station receiving a multicast framewhen the multicast frame is transmitted using a directional antenna.

Solution to Problem

In an aspect, a method of transmitting a multicast frame in a wirelesscommunication system is provided. The method comprises transmitting arequest to send (RTS) frame to stations (STAs) included in a multicastgroup by using an omni-directional antenna, and receiving a clear tosend (CTS) frame transmitted by the STAs included in the multicast groupin response to the RTS frame, and transmitting the multicast frame tothe STAs included in the multicast group by using a directional antenna.

The RTS frame may comprise information indicating an antenna mode to beused in transmission of the multicast frame.

The multicast frame may comprise a retransmission indicator field toprovide information indicating whether the multicast frame isretransmitted.

The multicast frame may comprise a retransmission indicator field toprovide information indicating whether a multicast frame transmittedimmediately before transmission of the multicast frame is retransmitted.

In another aspect, a method of transmitting a multicast frame in awireless communication system is provided. The method comprisestransmitting a CTS-to-self frame which comprises a receiver address, atransmitter address, and a multicast group address and in which thereceiver address is the same as the transmitter address, andtransmitting the multicast frame to an STA included in a multicast groupindicated by the multicast group address by using a directional antenna.

The CTS-to-self frame may further comprise information on an antennamode to be used in transmission of the multicast frame.

The multicast frame may comprise a retransmission indicator field toprovide information indicating whether the multicast frame is scheduledto be retransmitted.

The multicast frame may comprise a retransmission indicator field toprovide information indicating whether a preceding multicast frametransmitted immediately before transmission of the multicast frame isscheduled to be retransmitted.

Advantageous Effects of Invention

According to the present invention, a wireless local area network (WLAN)can not only expand service coverage but also ensure reliability ofmulticast transmission.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing an exemplary structure of a very highthroughput (VHT) wireless local area network (WLAN) system according toan embodiment of the present invention.

FIG. 2 is a message flow diagram of a multicast procedure according toan embodiment of the present invention.

FIG. 3 is a message flow diagram of a multicast procedure according toanother embodiment of the present invention.

FIG. 4 is a timing diagram corresponding to a message flow diagram ofFIG. 3 according to an embodiment of the present invention.

FIG. 5 is a timing diagram of a multicast procedure according to anembodiment of the present invention.

FIG. 6 shows a structure of an institute of electrical and electronicsengineers (IEEE) 802.11 medium access control (MAC) frame.

FIG. 7 shows an example of a method of using a retry bit to detect aframe received in a duplicate manner according to an embodiment of thepresent invention.

FIG. 8 shows an example of a method of using a retry bit to detect aframe received in a duplicate manner according to another embodiment ofthe present invention.

FIG. 9 is a block diagram of a station supporting a transmission methodof the present invention.

MODE FOR THE INVENTION

Embodiments of the present invention described below can effectivelyapply to a multicast procedure of a very high throughput (VHT) wirelesslocal area network (WLAN) system operating in a band of 60 GHz. However,the present invention is not limited thereto. For example, theembodiments of the present invention can also equally apply to a VHTWLAN system operating in a band of 6 GHz or lower.

In addition, in multicast, a target address or a destination address ofa transmit (Tx) frame is specified as a multicast group address.Broadcast is special multicast in which the multicast group addressspecifies all terminals. Therefore, when simply referred to as‘multicast’ in the following description, it will be interpreted that‘broadcast’ is also included unless it is not allowed by nature.

FIG. 1 is a schematic view showing an exemplary structure of a VHT WLANsystem according to an embodiment of the present invention.

Referring to FIG. 1, a WLAN system such as the VHT WLAN system includesone or more basis service sets (BSSs). The BSS is a set of stations(STAs) which are successfully synchronized to communicate with oneanother, and is not a concept indicating a specific region. As in theWLAN system to which the embodiment of the present invention isapplicable, a BSS that supports a super high-rate data processing of 1GHz or higher in a medium access control (MAC) service access point(SAP) is referred to as a VHT BSS.

The VHT BSS can be classified into an infrastructure BSS and anindependent BSS (IBSS). The infrastructure BSS is shown in FIG. 1.Infrastructure BSSs (i.e., BSS1 and BSS2) include one or more non-accesspoint (AP) STAs (i.e., Non-AP STA1, Non-AP STA3, and Non-AP STA4), APSTAs (i.e., AP STA1 and AP STA2) which are STAs providing a distributionservice, and a distribution system (DS) connecting the plurality of APSTAs (i.e., AP STA1 and AP STA2). In the infrastructure BSS, an AP STAmanages non-AP STAs of the BSS.

On the other hand, the IBSS is a BSS operating in an ad-hoc mode. Sincethe IBSS does not include the VHT STA, a centralized management entityfor performing a management function in a centralized manner does notexist. That is, the IBSS manages the non-AP STAs in a distributedmanner. In addition, in the IBSS, all STAs may consist of mobile STAs,and a self-contained network is configured since access to the DS is notallowed.

The STA is an arbitrary functional medium including a medium accesscontrol (MAC) and wireless-medium physical layer interface conforming tothe institute of electrical and electronics engineers (IEEE) 802.11standard, and includes both an AP and a non-AP STA in a broad sense. AVHT STA is defined as an STA that supports the super high-rate dataprocessing of 1 GHz or higher in the multi-channel environment to bedescribed below. In the VHT WLAN system to which the embodiment of thepresent invention is applicable, STAs included in the BSS may be all VHTSTAs, or a VHT STA and a legacy STA (i.e., IEEE 802.11n-based HT STA)may coexist.

The STA for wireless communication includes a processor and atransceiver, and also includes a user interface, a display means, etc.The processor is a functional unit devised to generate a frame to betransmitted through a wireless network or to process a frame receivedthrough the wireless network, and performs various functions to controlSTAs. The transceiver is functionally connected to the processor and isa functional unit devised to transmit and receive a frame for the STAsthrough the wireless network.

Among the STAs, non-AP STAs (i.e., STA1, STA3, STA4, and STA5) areportable terminals operated by users. A non-AP STA may be simplyreferred to as an STA. The non-AP STA may also be referred to as aterminal, a wireless transmit/receive unit (WTRU), a user equipment(UE), a mobile station (MS), a mobile terminal, a mobile subscriberunit, etc. A non-AP VHT-STA (or simply VHT STA) is defined as a non-APSTA that supports the super high-rate data processing of 1 GHz or higherin the multi-channel environment to be described below.

The AP (i.e., AP1 and AP2) is a functional entity for providing accessto the DS through a wireless medium for an associated STA. Althoughcommunication between non-AP STAs in an infrastructure BSS including theAP is performed via the AP in principle, the non-AP STAs can performdirect communication when a direct link is set up. In addition to theterminology of an access point, the AP may also be referred to as acentralized controller, a base station (BS), a node-B, a basetransceiver system (BTS), a site controller, etc. A VHT AP is defined asan AP that supports the super high-rate data processing of 1 GHz orhigher in the multi-channel environment to be described below.

A plurality of infrastructure BSSs can be interconnected by the use ofthe DS. An extended service set (ESS) is a plurality of BSSs connectedby the use of the DS. STAs included in the ESS can communicate with oneanother. In the same ESS, a non-AP STA can move from one BSS to anotherBSS while performing seamless communication.

The DS is a mechanism whereby one AP communicates with another AP. Byusing the DS, an AP may transmit a frame for STAs associated with a BSSmanaged by the AP, or transmit a frame when any one of the STAs moves toanother BSS, or transmit a frame to an external network such as a wirednetwork. The DS is not necessarily a network, and has no limitation inits format as long as a specific distribution service specified in theIEEE 802.11 can be provided. For example, the DS may be a wirelessnetwork such as a mesh network, or may be a physical construction forinterconnecting APs.

In a case where a VHT AP and/or a VHT STA use an omni-directionalantenna in the VHT WLAN system, a problem may arise in that transmissionreliability may not be able to be ensured in multicast transmission. Thereason above is that, when multicast transmission is used under thepremise that a plurality of VHT STAs simultaneously receive data, theplurality of VHT STAs are located in several places in a distributedmanner and thus some VHT STAs may be located in a transmitter device(e.g., VHT AP) for multicasting a media stream whereas some other VHTSTAs may be located in a long distance from each other. As describedabove, due to narrow service coverage of the VHT WLAN system, it is verydifficult to ensure transmission reliability of a multicast media streamwhen the VHT STA is located far from the VHT AP (i.e., transmitterdevice) or when the VHT STA is located in a non-line of sight (NLOS) ofthe VHT AP. Such a problem is a significant issue when the VHT WLANsystem operates in the band of 60 GHz due to its narrow servicecoverage. The same problem may also occur when the VHT WLAN systemoperates in the band of 6 GHz or lower.

To solve this problem, a method of using a directional antenna can betaken into consideration. The use of the directional antenna can improvenot only service coverage but also transmission reliability. For this,however, a multicast service of the VHT WLAN system requires a protocolfor using the directional antenna. Hereinafter, the protocol for usingthe directional antenna in the multicast service of the VHT WLAN systemwill be described.

The followings are assumed in an embodiment of the present inventiondescribed below.

First, a device for transmitting a media stream to be multicast, e.g., aVHT AP, supports transmission using a directional antenna (e.g.,transmission using beamforming). This implies that the VHT AP cantransmit a multicast stream in an omni-directional mode or a directionalmode. When the VHT AP transmits the multicast stream in theomni-directional mode, VHT STAs adjacent to the VHT AP can successfullyreceive the multicast stream, whereas VHT STAs located in an NLOS cannotsuccessfully receive the multicast stream. On the other hand, when theVHT AP transmits the multicast stream in the directional mode, VHT STAslocated in a corresponding direction as well as the VHT STAs located inthe NLOS can successfully receive the multicast stream.

According to the embodiment of the present invention, it is notnecessary for the VHT STA receiving the multicast stream to support thedirectional antenna. That is, although some VHT STAs support receptionusing the directional antenna, other VHT STAs may not support thereception using the directional antenna. However, when the VHT APtransmits the multicast stream in the directional mode, the VHT STAssupporting the reception using the directional antenna can coordinatebeamforming (BF) reception.

Under this assumption, the multicast frame can be transmitted andreceived according to four transmission/reception modes as following: 1)transmission in the omni-directional mode and reception in theomni-directional mode; 2) transmission in the omni-directional mode andreception in the directional mode; 3) transmission in the directionalmode and reception in the omni-directional mode; and 4) transmission inthe directional mode and reception in the directional mode. The VHT WLANsystem can use any one of the four transmission/reception modes or twoor more combinations of the four modes. According to the embodiment ofthe present invention, transmission in the omni-directional mode andtransmission in the directional mode are both used together in order toexpand service coverage of multicast transmission and to ensuretransmission reliability. In addition, VHT STAs receiving the multicaststream preferably coordinate the reception mode according to thetransmission mode, but the present invention is not limited thereto.

FIG. 2 is a message flow diagram of a multicast procedure according toan embodiment of the present invention based on the aforementionedassumption.

First, a VHT AP transmitting a multicast frame determines a VHT STAtransmitting the multicast frame by using an omni-directional mode and aVHT STA transmitting the multicast frame by using a directional modeamong VHT STAs registered to a multicast group, i.e., a VHT STA1, a VHTSTA2, and a VHT STA3. The VHT AP can determine a transmit (Tx) antennamode to be used when the multicast frame is transmitted to each VHT STAaccording to a specific criterion. For example, the VHT AP can determinethe Tx antenna mode based on a location of the VHT STA. The location ofthe VHT STA is a relative location with respect to the VHT AP, and maybe in association with a distance from the VHT STA to the VHT AP or maydepend on whether the VHT STA is located in an NLOS of the VHT AP.

For example, when all VHT STAs are located adjacent to the VHT AP orlocated in a line of sight (LOS), the VHT AP may determine to use onlythe omni-directional mode for the transmission of the multicast frame.Alternatively, when some VHT STAs are located adjacent to the VHT AP (orlocated in an LOS) and other VHT STAs are located far from the VHT AP(or located in an NLOS), it may be determined such that the former VHTSTAs transmit the multicast frame by using the omni-directional mode andthe latter VHT STAs transmit the multicast frame by using thedirectional mode. Alternatively, when all VHT STAs are located far fromthe VHT AP (or located in the NLOS), the VHT AP may determine to useonly the directional mode for the transmission of the multicast frame.

According to the embodiment of the present invention, the VHT APdetermines to use the omni-directional mode and/or the directional modeto transmit the multicast frame, and thereafter reports the determinedTx antenna mode to the VHT STA. This is for allowing VHT STAs receivingthe multicast frame to be able to use a receive (Rx) antenna mode in aproper manner according to the Tx antenna mode of the multicast frame.The reason above is that optimal beamforming for an Rx antenna of a VHTSTA is differently configured for a case where the VHT AP transmits themulticast frame in the omni-directional mode and a case where the VHT APtransmits the multicast frame in the directional mode. For example, in acase where the VHT AP transmits the multicast frame by using thedirectional mode with respect to a third terminal (i.e., VHT STA3), thethird terminal also adjusts an Rx antenna with respect to the VHT AP inaccordance with transmission in the directional mode.

In addition, according to the embodiment of the present invention, in acase where the VHT AP determines to transmit the multicast frame byusing the directional mode, other VHT STAs not registered to themulticast group (e.g., a fourth terminal (i.e., VHT STA4)) need to beprevented from accessing to a channel. The reason above is that, in acase where the multicast frame is transmitted using the directionalmode, a VHT STA located in a different region may recognize that thechannel is in an idle state according to a sensing mechanism of the VHTSTA itself.

In order to achieve such a purpose, prior to the transmission of themulticast frame, the VHT AP exchanges a request to send (RTS)frame/clear to send (CTS) frame with the VHT STAs registered to themulticast group or broadcasts a CTS-to-self frame. Exchanging of the RTSframe/CRS frame or broadcasting of the CTS-to-self frame is performed toreport a transfer mechanism of a multicast frame and, as to otherterminals not registered to the multicast group, to configure a networkallocation vector (NAV) during a time for transmitting the multicastframe. A procedure of exchanging the RTS frame and the CTS frame isshown in FIG. 2, and an embodiment of using the CTS-to-self frame willbe described later.

First, the VHT AP transmits an RTS frame prior to transmission of amulticast frame (step S11). The RTS frame has a format typically used inthe related art, and further includes a Tx antenna mode field (or simplyTx mode field) for reporting which one is used between theomni-directional mode or the direction mode to transmit the multicastframe. For example, the RTS frame may include at least a transmitteraddress field, a receiver address field, a multicast address field, andthe Tx mode field.

The transmitter address field may be set to a medium access control(MAC) address of a device transmitting the RTS frame, e.g., VHT AT. Thereceiver address field may be set to an MAC address of an STA receivingthe RTS frame, e.g., VHT STA. The VHT STA may be an STA selected by aVHT AP that is a device transmitting the RTS frame. The multicastaddress field may be set to an address of a multicast group forreceiving a multicast frame to be transmitted after exchange of the RTSframe and the CTS frame.

The Tx mode field is for setting of a Tx antenna mode of a multicastframe to be transmitted after exchange of the RTS frame and the CTSframe. The Tx mode field may be set to a value indicating theomni-directional mode or a value indicating the directional mode. The Txantenna mode may be set individually for each VHT STA receiving themulticast frame. For example, a VHT STA1, a VHT STA2, and a VHT STA3 maybe all set to the directional mode, and information indicating suchsetting may be included in the Tx mode field.

Referring continuously to FIG. 2, among VHT STAs which have received theRTS frames, VHT STAs registered to the multicast group transmit CTSframes to the VHT AP in response to the RTS frames (step S12). There isno particular restriction on an order of transmitting each CTS frame bythe VHT STAs. Thus, the CTS frames may be transmitted to the VHT APeither simultaneously or sequentially according to a specifictransmission protocol. The CTS frame also has a format typically used inthe related art. However, according to an embodiment, the CTS frame mayfurther include an Rx antenna mode field (or simply Rx mode field) to beused by the VHT STA to receive the multicast frame. For example, the CTSframe includes at least a receiver address field and a multicast addressfield, and may further include the Rx mode field.

The receiver address field may be set to an MAC address of an STAreceiving a CTS frame, e.g., VHT AP. Herein, the VHT AP is a devicetransmitting an RTS frame. The multicast address field may be set to anaddress of a multicast group for receiving a multicast frame to betransmitted after exchange of the RTS frame and the CTS frame. The Rxmode field is for setting an Rx antenna mode of a multicast frame to betransmitted after exchange of the RTS frame and the CTS frame. The Rxmode field may be set to a value indicating the omni-directional mode ora value indicating the directional mode. The Rx antenna mode may reportinformation regarding beamforming performed by a VHT STA transmitting aCTS frame to receive the multicast frame in a subsequent process.

Meanwhile, among VHT STAs which have received the RTS frame, VHT STAsnot registered to the multicast group (e.g., VHT STA4) configure an NAVuntil a time for transmitting a subsequent multicast frame. That is, theVHT STA4 does not attempt to access to a channel during a time when theVHT AP transmits the multicast frame.

Referring continuously to FIG. 2, the VHT STAs which belong to themulticast group and intend to receive a multicast frame in a subsequentprocess perform beamforming suitable for Tx antenna modes respectivelyreceived (step S13). For example, each of the VHT STA1, the VHT STA2,and the VHT STA3 may perform beamforming of Rx antennas for the VHT AP.

The VHT AP transmits the multicast frame to each VHT STA (step S14). Inthis case, the VHT AP may transmit the multicast frame in either theomni-directional mode or the directional mode to each VHT STA inresponse to information included in the RTS frame in step S11. Althoughit is shown in FIG. 2 that the multicast frame is transmitted to all VHTSTAs (i.e., VHT STA1, VHT STA2, and VHT STA3), this is for exemplarypurposes only.

FIG. 3 is a message flow diagram of a multicast procedure according toanother embodiment of the present invention based on the aforementionedassumption. In this embodiment, in order to report to VHT STAs a Txantenna mode to be used in the transmission of the multicast frame, adevice transmitting a multicast frame, e.g., VHT AP, broadcasts aCTS-to-self frame to a VHT STA instead of using a process of exchangingan RTS frame and a CTS frame with VHT STAs. In this sense, the presentembodiment differs from the aforementioned embodiment. The remainingprocesses other than that can use the aforementioned embodiment withoutany change. The following description will focus on a different aspectwith respect to the aforementioned embodiment.

First, among VHT STAs registered to a multicast group (e.g., VHT STA1,VHT STA2, and VHT STA3), a VHT AP transmitting a multicast framedetermines a VHT STA transmitting a multicast frame by using anomni-directional mode and a VHT STA transmitting a multicast frame byusing a directional mode. The VHT AP may determine a Tx antenna mode tobe used for the transmission of the multicast frame with respect to eachVHT STA according to a specific rule. In the present embodiment, the VHTAP determines that the VHT STA1 and the VHT STA2 use theomni-directional mode and that the VHT STA3 uses the directional mode,which differs from the previous embodiment in which all VHT STAs use thedirectional mode.

Further, the VHT AP transmits a CTS-to-self frame prior to thetransmission of the multicast frame (step S21). The CTS-to-self framehas a format typically used in the related art, and further includes aTx antenna mode field (or simply Tx mode field) for reporting which oneis used between the omni-directional mode or the direction mode totransmit the multicast frame. For example, the CTS-to-self frame mayinclude at least a receiver address field, a multicast address field,and the Tx mode field.

The receiver address field may be set to an MAC address of an STAtransmitting the CTS-to-self frame, i.e., VHT AP. The STA transmittingthe CTS-to-self frame is identical to an STA receiving the CTS-to-selfframe. The multicast address field may be set to an address of amulticast group for receiving a multicast frame to be transmitted afterthe transmission of the CTS-to-self frame. The Tx mode field is forsetting the Tx antenna mode of the multicast frame to be transmittedafter the transmission of the CTS-to-self frame. The Tx mode field maybe set to a value indicating the omni-directional mode or a valueindicating the directional mode. The Tx antenna mode may be setindividually for each VHT STA receiving the multicast frame. Forexample, the VHT STA1 and the VHT STA2 may be set to theomni-directional mode, and the VHT STA3 may be set to the directionalmode.

Referring continuously to FIG. 3, the VHT STAs which belong to themulticast group and intend to receive a multicast frame in a subsequentprocess perform beamforming suitable for respective Tx antenna modesreceived by being included in the CTS-to-self frame (step S22). Forexample, the VHT STA1 and the VHT STA2 may perform beamforming of an Rxantenna with respect to a VHT AP according to the omni-directional mode,and the VHT STA3 may perform beamforming of the Rx antenna with respectto the VHT AP according to the directional mode.

The VHT AP transmits the multicast frame to each VHT STA (step S23). Inthis case, the VHT AP may transmit the multicast frame in either theomni-directional mode or the directional mode to each VHT STA inresponse to information included in the CTS-to-self frame in step S21.Although it is shown in FIG. 3 that the multicast frame is transmittedto the VHT STA1 and the VHT STA2 in the omni-directional mode and istransmitted to the VHT STA3 in the directional mode, this is forexemplary purposes only.

FIG. 4 is a timing diagram corresponding to the message flow diagram ofFIG. 3 according to an embodiment of the present invention. Referring toFIG. 4, after a specific contention period elapses, a transmissionopportunity (TXOP) is given to a VHT AP. During the given TXOP, the VHTAP first transmits a CTS-to-self frame in an omni-directional mode. TheCTS-to-self frame reports Tx antenna modes to be applied respectively toterminals belonging to a multicast group (i.e., VHT STA1, VHT STA2, andVHT STA3), or allows a 3rd party terminal not belonging to the multicastgroup (i.e., 3rd part VHT STA) to configure an NAV during a specificperiod including a transfer period. In addition, the VHT AP transmits amulticast frame (i.e., data) to each VHT STA during the transfer periodin the omni-directional mode and/or a directional mode. In this case,the data may be repetitively transmitted so that the data can bereceived by all VHT STAs registered to the multicast group regardless ofwhether the VHT STAs are located in an LOS or an NLOS.

The aforementioned embodiment of the present invention can also apply toa situation where a coordinator such as a VHT AP does not exist in a VHTWLAN system.

In the VHT WLAN system not employing the coordinator such as the VHT AP,e.g., an independent BSS or the like, a VHT STA transmitting a multicastframe, e.g., a source VHT STA, cannot transmit the multicast frame tothe VHT AP in a unicast manner. Therefore, the source VHT STA has todirectly multicast/broadcast the multicast frame without the aid ofother elements. For this, before the transmission of the multicastframe, the source VHT STA may exchange an RTS frame and a CTS frame ortransmit a CTS-to-self frame so that the VHT STAs can performbeamforming for a directional antenna. The RTS/CTS frame or theCTS-to-self frame can be used in the same manner as described in theaforementioned embodiment.

FIG. 5 is a timing diagram of a multicast procedure according to anembodiment of the present invention when there is no coordinator such asa VHT AP. Referring to FIG. 5, a source VHT STA transmits a CTS-to-selfframe prior to transmission of a multicast frame. The CTS-to-self framemay include a receiver address field, a multicast address field, and aTx mode field. The receiver address field may be set to an MAC addressof a STA transmitting the CTS-to-self frame, i.e., a source VHT STA. Themulticast address field may be set to an address of a multicast groupfor receiving a multicast frame to be transmitted after the transmissionof the CTS-to-self frame. The Tx mode field is for setting a Tx antennamode of the multicast frame to be transmitted after the transmission ofthe CTS-to-self frame. The Tx mode field may be set to a valueindicating the omni-directional mode or a value indicating thedirectional mode. The Tx antenna mode may be set individually for eachVHT STA receiving the multicast frame. In a case where another VHT STAwhich has listened the CTS-to-self frame desires to receive a multicastframe of a multicast group specified in the multicast address field,beamforming suitable for the Tx antenna mode can be performed withrespect to the source VHT STA which has transmitted the CTS-to-selfframe.

According to the aforementioned embodiment of the present invention, theVHT AP transmitting a multicast frame or the source VHT STA cancontinuously transmit the multicast frame during a specific transferperiod (e.g., a duration given by TXOP). In a case where the multicastframe is contiguously transmitted, a Tx antenna mode for a specific VHTSTA may be fixed or changeable. For example, if a VHT STA1 moves to adifferent location, for the VHT STA1, the multicast frame may betransmitted first in the omni-directional mode and then in thedirectional mode, or may be transmitted first in the directional modeand then in the omni-directional mode.

If there is a change in the Tx antenna mode, the VHT AP needs to reportthis to a corresponding VHT STA. As one method for reporting this,information on a Tx antenna mode to be used in transmission of a nextmulticast frame is included in a specific part of a multicast frame(e.g., an MAC header of the multicast frame). That is, the VHT STA canperform beamforming according to an antenna mode used in thetransmission of the next multicast frame by using the Tx antenna modeinformation included in the received multicast frame.

Meanwhile, in a case where a VHT AP or a source VHT STA transmits amulticast frame to one or more VHT STAs in the directional mode, thesame multicast frame may be repetitively transmitted. For example, thesame multicast frame may be transmitted in the omni-directional mode inone time and then transmitted in the directional mode in a next time toa specific VHT STA. Alternatively, the same multicast frame may betransmitted in the directional mode to each VHT STA. In this case, thespecific VHT STA may redundantly receive the same multicast frame, andthus a process of filtering the redundantly received multicast frame maybe required for the VHT STA.

The filtering of the redundantly received frame may be performed byusing a sequence control field and a traffic identifier (TID) subfieldincluded in the multicast frame. The sequence control field includes asequence number and a fragment number. An MAC protocol data unit (MPDU),which is a part of the same MAC service data unit (MSDU), has the samesequence number. However, there is a high probability that differentMSDUs have different sequence numbers. In addition, the sequence numberfor the multicast frame can be generated from different counters withrespect to each TID and multicast/receiver address pair. This sequencenumber may be incremented by one with respect to each new MSDUcorresponding to the TID and multicast address pair. Instead of usingthe TID and receiver address pair, a TID and transmitter address pairmay be also used.

Hereinafter, a method of detecting a multicast frame received in aduplicate manner according to an embodiment of the present inventionwill be described in greater detail.

FIG. 6 shows a structure of an IEEE 802.11 MAC frame. In a detectionmethod described below, a retransmission indicator field may be added toprovide information indicating whether the multicast frame with the MACframe structure of FIG. 6 is retransmitted, or a reserved bit of theconventional field may be used. In addition, a field unused in theconventional multicast frame transmission may be used.

As one of subfields constituting a frame control field in theconventional MAC frame structure, a retry field 600 is used in theembodiment of the present invention. However, the present invention isnot limited thereto, and thus a newly added retransmission indicatorfield may be used in the same manner as the method to be describedbelow, or a reserved bit may be used in the implementation of thepresent invention.

The retry field has a length of one bit. As for an initially transmittedframe, a retry bit which is a value of the retry field is set to 0 intransmission. However, when the frame is retransmitted, the retry bit isset to 1 in the retransmitted frame, and thus it is helpful for arecipient STA to remove a duplicate frame.

When an AP relays a multicast frame by using the conventional method, anopen loop transmission scheme is used so that the recipient STA does nottransmit any feedback, including an acknowledgement (ACK) frame, inresponse to a received multicast frame. Therefore, in the conventionalmethod of transmitting a multicast frame in an omni-directional mode,the multicast frame is not retransmitted, and the retry bit is alwaysset to 0 in transmission of the multicast frame.

In a method of transmitting a multicast frame according to an embodimentof the present invention, the multicast frame is transmitted using bothan omni-directional mode and a directional mode. In this embodiment, aVHT AP can utilize a retry field of the multicast frame to detect aduplicate frame.

In the method of transmitting the multicast frame according to theembodiment of the present invention, the VHT AP configures the retry bitof the multicast frame in the following manner. When the VHT APtransmits the multicast frame, if the multicast frame to be transmittedis a frame to be retransmitted to another STA at a later time, the retrybit is set to 1. If the multicast frame to be transmitted is not themulticast frame to be retransmitted at the later time, the retry bit isset to 0.

The multicast frame transmitted by setting the retry bit to 1 may beretransmitted by the VHT AP, and thus is stored in a memory (e.g., abuffer, a cache, etc.) so as to be used in frame duplication detection.The memory for storing the multicast frame is not limited to the buffer,the cache, etc., and thus all data storage media capable of temporarilystoring the multicast frame can be used.

The multicast frame of which the retry bit is set to 0 is not stored inthe memory since there is no possibility of retransmitting the multicastframe by the VHT AP. In addition, as for the multicast frame of whichthe retry bit is set to 0, if there are multicast frames which arestored in the memory and which have the same recipient address whilehaving a small sequence number, those multicast frames are deleted fromthe memory.

For this, the VHT AP may update the sequence number of the multicastframes in the following manner. The VHT AP sequentially increments thesequence number of the multicast frames for each pair of (multicastaddress, TID) by using respective counters. In this case, the number ofthe counters corresponds to the number of multicast addresses supported.

As another example of the update method, the VHT AP sequentiallyincrements the sequence number of the multicast frames for each pair of(transmitter address, TID) by using the respective counters. In thiscase, the number of the counters corresponds to the number oftransmitter addresses.

As still another example of the update method, the VHT AP sequentiallyincrements the sequence number of the multicast frames for each pair of(multicast address, transmitter address, TID) by using the respectivecounters. In this case, the number of counters corresponds tocombinations of the number of multicast addresses and the number oftransmitter addresses.

If the retry bit of the received multicast frame is 1, the VHT STAstores a multicast address, a transmitter address, a TID, a sequencenumber, and a fragment number in a memory as one tuple. Otherwise, ifthe retry bit is 0, the VHT STA does not store these elements.

FIG. 7 shows an example of a method of using a retry bit to detect aframe received in a duplicate manner according to the present invention.

It is assumed herein that a VHT STA1 operates in an active mode, and aVHT STA2 and a VHT STA3 operate in a power save mode. It is also assumedthat a VHT AP transmits a multicast frame to each VHT STA in adirectional mode. However, this is for exemplary purposes only. Thus,three or more non-AP STAs may operate either in the active mode or thepower save mode, and the VHT AP may transmit the multicast frame to eachSTA or to an STA group by using an omni-directional mode together withthe directional mode.

If the VHT AP transmits a multicast frame group 700, of which a retrybit is set to 1, to the VHT STA1 in the directional mode, the VHT STA1receiving the multicast frame group stores a multicast address, atransmitter address, a TID, a sequence number, and a fragment number ofa multicast frame group 703, of which a retry bit is set to 1, in amemory as one tuple. Thereafter, when the VHT AP sequentially transmitsmulticast frame groups 710 and 720 to the VHT STA2 and the VHT STA3,respectively, since the VHT STA1 operates in the active mode in thepresent embodiment, the multicast frame group 710 transmitted toward theVHT STA2 is received as a multicast group 713. In this case, the contentof the multicast frame group 713 received by the STA1 is the samecontent as the previously received multicast frame group 703, which isduplicate reception. Similarly, the multicast frame group 720transmitted toward the VHT STA3 is received as a multicast frame group723, which is also duplicate reception. However, the VHT STA1 may detectand discard a duplicate frame by using information stored in the memory.

When transmitting the multicast frame group 720 toward the VHT STA3, acorresponding multicast frame group is not transmitted later, and thus aretry bit is set to 0 in all multicast frames included in the multicastframe group. The VHT STA1 and the VHT STA3 receiving correspondingframes delete information related to those frames from the memory.

In a method of transmitting a multicast frame according to anotherembodiment of the present invention, the VHT AP may set a retry bit ofthe multicast frame in the following manner.

When the VHT AP intends to retransmit a previous frame of a currentlytransmitted multicast frame, a retry bit of the currently transmittedmulticast frame is set to 1. When the previous frame of the currentlytransmitted multicast frame is not intended to be retransmitted, theretry bit of the currently transmitted frame is set to 0. In otherwords, the retry bit of the transmitted frame delivers informationindicating whether the VHT AP will retransmit a multicast frametransmitted immediately before the currently transmitted multicastframe. The information indicating whether the currently transmittedmulticast frame will be retransmitted is transmitted by being containedin a retry bit of a multicast frame to be transmitted immediately nextto the currently transmitted multicast frame.

Upon receiving the multicast frame, the VHT STA stores the currentlyreceived multicast frame in the memory regardless of the retry bit, anduses the stored multicast frame to determine whether a frame receivedlater is a duplicate frame. If the retry bit is set to 1 in the receivedmulticast frame, a multicast frame received immediately before has apossibility of being retransmitted by the VHT AP, and thus is stillstored in the memory. If the retry bit is set to 0 in the receivedmulticast frame, the multicast frame received immediately before has nopossibility of being retransmitted by the VHT AP, and thus is deletedfrom the memory. In addition, previously received multicast frames arealso deleted since they have no possibility of being retransmitted.

For a received multicast frame, the VHT STA stores a multicast address,a transmitter address, a TID, a sequence number, and a fragment numberin the memory as one tuple. If a retry bit of a transmitted multicastframe is 1, no action is taken. Otherwise, if the retry bit is 0, forthe received multicast frame, information which is stored in the memoryand which has the same pair of (multicast address, transmitter address,TID) while having a small sequence number is deleted from the memory.

FIG. 8 shows a detailed example of a method of using the aforementionedretry bit in a duplication detection of a received frame.

FIG. 8 differs from FIG. 7 in that retry bits of multicast frames 800and 810 with a sequence number 1 and multicast frames 830 and 840 with asequence number 5 are set to 1 in FIG. 7 whereas the retry bits thereofare set to 0 in FIG. 8. The retry bits of the multicast frames 800, 810,and 820 with the sequence number 1 contain information indicatingwhether to retransmit a multicast frame transmitted before the multicastframe with the sequence number 1. However, since there is no multicastframe transmitted before the multicast frame with the sequence number 1,the retry bit is set to 1.

Retry bits of multicast frame 830, 840, and 850 are set to 0 since apreviously transmitted multicast frame with a sequence number 4 is nolonger transmitted at a time when the multicast frame with the sequencenumber 5 is transmitted. Upon receiving the multicast frame which hasthe sequence number 5 and of which the retry bit is set to 0, VHT STAsdelete multicast frames which have the sequence numbers 1, 2, 3, and 4and which are stored in a memory. That is, when a VHT STA1 receives amulticast frame 833, when a VHT STA2 receives a multicast frame 846, andwhen a VHT STA3 receives a multicast frame 859, the multicast frameswhich have the sequence numbers 1, 2, 3, and 4 and which are stored inrespective memories are deleted. With this method, regardless of whetherthe VHT STA operates in an active mode or a power save mode, the VHT STAcan delete information on the multicast frames 1 to 4 stored in thememory while receiving the multicast frame with the sequence number 5.Therefore, this method can be utilized regardless of an operation modeof a corresponding STA.

FIG. 9 is a block diagram showing an STA according to an embodiment ofthe present invention. An STA 900 includes a processor 910, a memory920, and a transceiver 930. The transceiver 930 transmits/receives aradio signal, and implements an IEEE 802 physical layer. The transceiver930 supports an omni-directional mode and a directional mode. Theprocessor 910 is coupled to the transceiver 930, and implements an IEEE802 MAC layer. The processor 910 can implement the aforementioned methodof transmitting a multicast frame, a method of detecting a multicastframe received in a duplicate manner, etc.

The processor 910 and/or the transceiver 930 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. The memory 920 may includea read-only memory (ROM), a random access memory (RAM), a flash memory,a memory card, a storage medium, and/or other equivalent storagedevices. When the embodiment of the present invention is implemented insoftware, the aforementioned methods can be implemented with a module(i.e., process, function, etc.) for performing the aforementionedfunctions. The module may be stored in the memory 920 and may beperformed by the processor 910. The memory 920 may be located inside oroutside the processor 910, and may be coupled to the processor 910 byusing various well-known means.

The invention claimed is:
 1. A method of transmitting a multicast framein a wireless communication system, the method comprising: transmitting,by a transmitting station to a receiving station, a clear-to-send (CTS)type frame to inform the receiving station about an impendingtransmission of multicast frames; transmitting, by the transmittingstation to the receiving station, a first multicast frame of themulticast frames by using a first antenna configuration aftertransmitting the CTS type frame; and transmitting, by the transmittingstation to the receiving station, a second multicast frame of themulticast frames by using a second antenna configuration that isdifferent from the first antenna configuration, wherein the secondmulticast frame is a copy of the first multicast frame, wherein thesecond multicast frame uses a sequence number that is the same as thefirst sequence number in order enable the receiving station to filterout a duplicated multicast frame, and wherein the CTS type frameincludes a transmitter address field and a transmissions mode field, theaddress field including an address of the transmitting station thattransmits the CTS type frame, the transmission mode field consisting ofa single bit indicating whether at least one multicast frame is to betransmitted by the transmitting station.
 2. The method of claim 1,wherein the first antenna configuration includes a first antenna weightvector and the second antenna configuration includes a second antennaweight vector.
 3. The method of claim 1, wherein the first antennaconfiguration relates to an omni-directional antenna and the secondantenna configuration relates to a directional antenna.
 4. The method ofclaim 1, wherein the first multicast frame includes a first retry bitthat is set to one, and the second multicast frame includes a secondretry bit that is set to one.
 5. The method of claim 1, wherein thewireless communication system is operated in a 60 GHz band.
 6. Themethod of claim 1, wherein the first multicast frame includes a mediumaccess control service data unit (MSDU), a first destination addressfield being set to a multicast address and a first sequence fieldincluding the first sequence number, and wherein the second multicastframe includes the MSDU, a second destination address field being set tothe multicast address and a second sequence field including the firstsequence number.
 7. The method of claim 1, wherein the CTS type framecomprises a Clear-To-Send (CTS)-to-self frame to the receiving station,the CTS-to-self frame being a CTS frame transmitted without receiving aRequest to Send (RTS) frame.
 8. The method of claim 1, wherein thetransmitting station is an access point.
 9. A device configured fortransmitting a multicast frame in a wireless communication system, thedevice comprising: a memory; and a processor operatively coupled withthe memory and configured to: transmit, to a receiving station, aclear-to-send (CTS) type frame to inform the receiving station about animpending transmission of multicast frames; transmit, to the receivingstation, a first multicast frame of the multicast frames to thereceiving station by using a first antenna configuration aftertransmitting the CTS type frame; and transmit, to the receiving station,a second multicast frame of the multicast frames to the receivingstation by using a second antenna configuration that is different fromthe first antenna configuration, wherein the second multicast frame is acopy of the first multicast frame, wherein the second multicast frameuses a sequence number that is the same as the first sequence number inorder to enable the receiving station to filter out a duplicatedmulticast frame, and wherein the CTS type frame includes a transmitteraddress field and a transmission mode field, the address field includingan address of a transmitting station that transmits the CTS type frame,the transmission mode field consisting of a single bit indicatingwhether at least one multicast frame is to be transmitted by thetransmitting station.